Most efficient format, as in the one that takes up the least amount of space? Listen, don't worry about space. 500 GB/1 TB drives are not that expensive and e-books don't take up that much space anyway. Maybe if you have several thousands of them... but even then it's still manageable (probably under 10 GB).

And why would you want to remove images? That just isn't right... For ePubs, I guess you could delete the content of the Images folder, or maybe the folder itself. But will it still be a valid ePub then? I don't know. The images were put there for a reason. Removing them is like making soup without any oil or salt. Have you ever tried it? It tastes awful.